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Edwin S. Lyman, Ph.D
Nuclear Control Institute
September 21, 1995

Behavior of Mixed-Oxide Fuel Under Transport Accident Conditions

Mixed-oxide (MOX) fuel rods for use in pressurized-water reactors (PWRs) consist of stacks of sintered ceramic fuel pellets composed of a mixture of uranium and plutonium dioxides (UO2-PuO2), clad in a thin layer of Zircaloy (a zirconium-based metallic alloy). The fuel rods are pressurized internally with helium fill gas.

A number of mechanisms exist which could result in a dispersal of plutonium from unirradiated MOX fuel rods in the event of a severe transport accident. Substantial releases could occur if the fuel rods were subjected to a high-velocity impact which breached the cladding, followed by a fire of moderate temperature (as low as 500C) of several hours' duration. Exposure of the fuel pellets at this temperature to oxygen flowing through the breach would cause a "significant comminution" (disintegration into small particles) of the fuel, as the UO2 in the matrix expands to the larger unit- cell U3O8 structure. These particles would contain plutonium in the same proportion on average as was present in the bulk fuel. A large fraction of these particles would be small enough to be easily dispersed in aerosol form by the flames of the fire.

It should also be noted that unirradiated MOX fuel rods fabricated from commercial-grade plutonium will contain significant amounts of Am-241, a highly toxic alpha- and gamma- emitter. Am-241 has been observed to be more dispersible from heated MOX fuel pellets than plutonium and also poses a safety hazard.

Even if the cladding is not grossly breached by the impact, thermal exposure alone could eventually cause it to rupture as a result of overpressurization.

Fuel melting, although less likely, cannot be excluded. Uranium dioxide fuel can melt at temperatures as low as 1700C due to the formation of low-melting compounds with the cladding. Plutonium dioxide has a lower melting point than uranium dioxide and, if it is not uniformly distributed in the MOX fuel, it is possible that localized melting of MOX fuel can occur at temperatures hundreds of degrees lower, which would be in the range of temperatures encountered in aviation fuel pool fires.

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